[enh] update CHANGELOG.md

CHANGELOG.md

+## v0.51.0 (03/04/2020)
+### Features
+* #798 - Complete workflow to send transaction that we can get back after one week:
+    * A selector allow to choose the lock condition of the output transaction
+    * Then, the transaction with special lock condition appears with an underline in the transaction history
+    * If you are the receiver, you can use the context menu action "Send as source" to send it to yourself
+    before the issuer get it back
+    * If you are the issuer, use the context menu action "Send as source" to send it to yourself after the delay
+    * Before the delay, you can check the lock condition in the transfer window to see the date when the transaction will be unlocked
+    * In Expert Mode (see Preferences), you can use any incoming transaction or dividend as source for your transfer
+
+### Enhancement
+* Add a legend for colors, italic and underlined display of transactions under transaction history
+* Upgrade dependencies to duniterpy 0.57.0
+
+### Bugs
+* Fix gen_translations.py running Qt4 lrelease in Makefile
+* Fix error parsing some WSP2 Head message because software version had a prefix (duniterpy fix)
+
+### CI/CD
+* Remove obsolete runner tag
+* Release job only on `master`, release_test job only on `dev` or `tags`
+
 ## v0.50.5 (21/03/2020)
 ### Bugs
 * Fix send money to pubkey with search user selected